1. Field of the Invention
This invention relates to the drilling of wells and more particularly to apparatus to be installed in a drill string to protect the drill string during drilling operations.
2. Description of the Prior Art
During the drilling of a well, the torque required to rotate the drill string may become excessive for any of a number of reasons such as wear of the drill bit, junk in the hole or highly fractured formations collapsing on the drill string. The excessive torque may cause twistoff of the drill string and result in an expensive fishing operation or abandonment of the hole. Because the torque is applied to the drill string throughout its length above the location at which the obstruction to rotation occurs and the obstruction is most frequently near the bottom of the borehole, any weakness in the drill string at any point along its full length above the obstruction increases the chances of a twistoff.
U.S. Pat. No. 1,923,132 of Witkin and U.S. Pat. No. 1,610,414 of Bernard et al describe a safety joint to be installed in and constitute a part of the drill string to prevent applying excessive torque to the drill string. The safety joint described in each of the patents includes a lower portion that is rotatably suspended from an upper portion. During drilling, relative rotation of the two portions of the sub is prevented by shear pins which are sheared when the torque required to rotate the drill string becomes excessive. The apparatus described in U.S. Pat. Nos. 1,610,414 and 1,923,132 provides protection for the drill string but does not allow any rotation of the drill bit after the shear pins have been broken. Sometimes rotation of the drill bit is necessary to clear the hole and for removal of the drill string after an obstruction to drilling is encountered.
U.S. Pat. No. 3,204,992 of Walker and U.S. Pat. No. 2,532,686 of Ware describe safety joints to be installed in a drill string or other tubing which allows separation of the drill string or tube at the safety joint in the event the drill string or tubing becomes stuck in the hole or the torque required to rotate the drill string becomes excessive. In the apparatus described in U.S. Pat. Nos. 2,532,686 and 3,204,992 it is necessary for the operators to recognize that the drill string or tubing has become stuck. The apparatus does not provide means for automatically preventing the application of excessive torque throughout the length of the drill string or tubing; consequently does not provide protection against twistoffs.